Over the last few decades, the electronics industry has undergone a revolution through the use of semiconductor technology to fabricate small, highly integrated electronic devices. As these devices become smaller, there is a need for forming increasingly narrow conductive lines and interconnects in these devices. Many of these conductive lines and interconnects are formed using metals, including, for example, aluminum or copper. A layer of the metal is often formed over the substrate and previously formed layers of the device and then the metal layer is patterned to form the conductive lines and interconnects. A standard patterning technique is photolithography, in which a photoresist is deposited over the layer of metal and a mask is used to expose the photoresist with light which is passed through the mask. The machines used today are referred to as steppers or scanners and the pattern created by the mask can be the same size as the mask or reduced by a factor of, for example, five or ten. After exposure to light, the photoresist is developed, for example, by rinsing in a developing solution. The metal layer can then be etched to remove the unwanted material according to the desired pattern.
The wavelength of light used to expose the photoresist will typically determine the minimum size of a feature that can be patterned. Current 0.25 .mu.m generation design rules call for patterning 0.4 .mu.m lines and spaces in the metal interconnect layers. The exposure wavelength for patterning the photoresist for 0.4 .mu.m design rules is typically 365 nm. For more aggressive gate layer design rules, requiring 0.25 .mu.m lines and spaces, the exposure wavelength is 248 nm.
Future CMOS devices are expected to have gatewidths of about 0.18 .mu.m and metal line widths and spaces of about 0.3 .mu.m. This will require light with shorter wavelengths. One convenient wavelength is 248 nm, available from a KrF laser. Furthermore, CMOS devices may be designed with 0.13 .mu.m or smaller gatewidths. These devices will require photoresist patterning with even shorter wavelengths, including, for example, the 193 nm line of an ArF laser.
One particular difficulty with patterning metal layers is the inherent reflectivity of the metal. High reflectivity can distort the mask image in the photoresist. In general, there are three phenomena which degrade the resist image on reflective surfaces, such as metal or silicon. First, standing waves can be generated within the resist in the vertical direction due to the constructive and destructive interference of the incident and reflected light. Standing waves in the resist profile produce vertical waves in the resist which can lead to etching problems. A second effect, referred to as "swing", occurs when there is a change in the resist thickness due to a topographical change in the underlying layers (e.g., a change in thickness of the underlying layers). The horizontal width of the line is affected due to the path length difference of light reflected from either side of the topographical feature. The third effect is called "reflective notching" and occurs when the topography of the underlying surface (e.g., a slope in the topography) causes the reflection of light at angles which are not perpendicular to the surface of the photoresist. The reflected light then exposes portions of the photoresist which results in the removal of inappropriate portions of the desired lines (i.e., notches in the lines). Thus, to properly expose the resist, the reflectivity of the substrate below the resist must be very low. Ideally, this reflectivity should be 3% or less.
Aluminum and copper are commonly used metals for conductive lines and interconnects in semiconductor devices. These metals have reflectivities of about 80-90%. Therefore, an anti-reflective structure is applied over the metal layer prior to applying the photoresist. One commonly used anti-reflective structure is titanium nitride (TiN), which has a low reflectivity at 365 nm. However, at 248 nm, the reflectivity of titanium nitride is much greater than 3% and the reflectivity of titanium nitride is even larger for 193 nm light.